The present invention relates to a powder mixture which is particularly low in oxygen content and which is useful as a starting powder for manufacturing an R-Fe-B based magnet containing R (at least one of the rare earth elements inclusive of Y), Fe, and B as the major components. The starting powder of the present invention comprises the following powders [A] and [B] blended at a predetermined ratio to give a magnet having a desired composition:
[A] an alloy powder which gives the principal phase (sometimes simply referred to hereinafter as a powder based on principal phase) composed mainly of an R.sub.2 Fe.sub.14 B hard magnetic phase and having been prepared by a direct reduction diffusion process; and PA0 [B] an intermetallic compound powder comprising a phase of an intermetallic compound between R and Fe or Co inclusive of an R.sub.3 Co phase (wherein Co may be partly or largely replaced by Fe), this intermetallic compound powder containing a higher amount of a rare earth element as compared with the principal phase powder. PA0 [C] an intermetallic compound powder comprising a phase of an intermetallic compound between R, Fe or Co and B inclusive of an R.sub.3 Co phase (wherein Co may be partly or largely replaced by Fe) and partly R.sub.2 (Fe Co).sub.14 B phase, this intermetallic compound powder containing a higher amount of a rare earth element as compared with the principal phase powder.
In addition, the starting powder of the present invention can comprise the above-mentioned powder [A] and the under-mentioned powder [C] blended at a predetermined ratio to give a magnet having a desired composition:
An R-Re-B based permanent magnet, which is described in JP-A-59-46008 (the term "JP-A-" as used herein signifies a "unexamined published Japanese patent application"), is a representative of high performance permanent magnets known at present. An R-Fe-B permanent magnet exhibits excellent magnetic properties owing to a texture comprising a principal (hard magnetic) phase of a tetragonal ternary compound and an R-rich phase; it yields a coercive force, iHc, of 25 kOe or higher and an energy product, (BH)max, of 45 MGOe or higher, which are both considerably higher as compared with those of a conventional high performance rare earth-cobalt (REC) magnet. There has also been proposed various types of R-Fe-B magnets which are varied in composition to meet the diversified demands regarding magnetic properties.
To produce a variety of R-Fe-B permanent magnets by powder metallurgy, i.e., by sintering a powder, an alloy powder having a predetermined composition for the magnets should be prepared at first. Such alloy powders are produced at present by an ingot-making process and crushing (as described in JP-A-60-63304 and JP-A-60-119701) which comprises melting a rare earth material having been subjected to electrolytic reduction, casting the melt in a casting mould to obtain an alloy ingot having the desired composition for the magnet, and crushing the ingot to give an alloy powder at a predetermined granularity. Otherwise, a direct reduction and diffusion process as described in JP-A-59-219494 and JP-A-60-77943 is employed, which comprises preparing directly the alloy powder of the desired composition for the magnet, using a rare earth oxide, an Fe powder, and the like as the starting powder.
The ingot-making and crushing process provides an alloy powder relatively low in oxygen content. In this method, oxidation prevention can be easily conducted at the primary crushing process, however, primary Fe crystals tend to form 17 easily and the R-rich phase segregate to grow into large grains.
The direct reduction and diffusion process is advantageous in that the steps such as melting and coarse grinding (primary crushing) which are included in the ingot-making and crushing can be omitted. However, the final powder is often obtained with the R.sub.2 Fe.sub.14 B principal phase being surrounded by the R-rich phase. Furthermore, since the R-rich phase is more finely and better dispersed than by the ingot-making and crushing process, the R-rich phase is susceptible to oxidation during the production process and therefore it contains a higher amount of oxygen. Accordingly, magnets of a certain composition suffer fluctuation in magnetic properties and the like, ascribed to the consumption of the rare earth elements.
The powder produced by a direct reduction and diffusion process is further advantageous in that the R-rich phase which surrounds the principal phase is relatively small. This signifies that the R-rich phase finely disperses as a liquid during the sintering, resulting in a dense magnet with a favorable squareness ratio.
As mentioned in the foregoing, an R-Fe-B based powder produced by a direct reduction and diffusion process for permanent magnets is advantageous in that it can be produced by a process in which steps of melting, coarse grinding, etc., can be omitted, and that it has a higher density with a favorable squareness ratio as a magnet. However, since the R-rich phase is finely and well dispersed in the powder thus produced, the powder becomes susceptible to oxidation and tends to contain a higher amount of oxygen as compared with a powder produced by an ingot-making and crushing process. This leads to a fluctuation in the magnetic characteristics of the final magnet due to a slight oxidation during the manufacturing process thereof.
It is possible to provide an intermetallic compound relatively stable against oxidation by adding elements such as Co and Ni to the R-rich phase and thereby reduce the oxygen content of the powder. However, it is not possible to optimally control the addition of such elements in such a manner to most effectively attain a predetermined composition.
That is, it is requisite that the amount of addition of one or plural rare earth elements is controlled to obtain magnetic properties as desired, and, if Co were to be added to reduce the oxygen content, the Co would diffuse not only into the R-rich phase as desired, but also into the principal phase to be included as a substituent for Fe.
Furthermore, though depending on the amount, the addition of elements such as Co and Ni reduces the coercive force of the magnet, and this is another point which makes it difficult to lower the oxygen content.
The starting powder for magnets prepared by both of the conventional processes, i.e., the ingot-making and crushing process and the direct reduction and diffusion process, is not a product obtained simply by effecting the process on a powder mixture having been blended to a desired composition depending on the required magnetic properties, but has a particular texture composed of a tetragonal ternary compound as the principal phase and an R-rich phase. Accordingly, the plural rare earth elements to be added should be controlled each to a predetermined content designed to obtain the desired alloy composition, so that it may yield the intended magnetic properties. Thus, the alloy composition and the compositional ratio should be always taken into consideration, for example, which rare earth element is more apt to be incorporated in the principal phase and which one is more likely to constitute the R-rich phase. This signifies that, to obtain magnetic properties as desired, the starting alloy powder should be prepared as such to give a specific composition being confined to an extremely narrow range.
In other words, it is very difficult to obtain a starting powder mixture having the metals and alloy powders blended exactly to the ratio of the desired magnet composition. To obtain a magnet having the characteristics as desired, there should be prepared a variety of alloy powders each differing in alloy structure and composition.